A DC power supply device including a DC/DC converter having FETs each driven by a drive transformer. A voltage from a single drive power supply disposed in common for the FETs is divided into positive and negative biases to be applied to the FETs, and an operational state of the FETs is detected based on voltage signals. A sequence circuit turns on an input from a three-phase AC power supply by driving a relay circuit at a time point when it is confirmed that the FETs have normally started stable ON/OFF operation, and drives a power factor improvement circuit, which converts AC voltage from the three-phase AC power supply into a DC voltage by simultaneously performing full-wave rectification and power factor improvement.